Unveiling the multilevel structure of midgap states in Sb-doped MoX2 (X=S, Se, Te) monolayers

In this study, we use DFT calculations to investigate the electronic and\nstructural properties of MoX$_2$ (X = S, Se, Te) monolayers doped with\nsubstitutional Sb atoms, with a central focus on the Sb(Mo) substitution. In\nMoS$_2$, we observe that this substitution is energetically favored under S\nrich conditions, where the S$_2$ gaseous phase is likely to be present. This\nresult is compatible with a recent experimental observation in Sb-doped MoS$_2$\nnanosheets grown by CVD. A similar behavior is found in MoSe$_2$, but in\nMoTe$_2$ the Sb(Mo) substitution is less likely to occur due to the possible\nabsence of gaseous Te phases in experimental setups. In all cases, several\nimpurity-induced states are found inside the band gap, with energies that span\nthe entire gap. The Fermi energy is pinned a few tenths of eV above the top of\nthe valence band, suggesting a predominant $p$-type behavior. The orbital\nnature of these states is investigated with projected and local density of\nstates calculations, which reveal similarities to defect states induced by\nsingle Mo vacancies as well as their rehybridization with the $5s$ orbital from\nSb. Additionally, we find that the band gap of the doped systems is increased\nin comparison with the pristine materials, in contrast with a previous\ncalculation in Sb-doped MoS$_2$ that predicts a gap reduction with a different\nassignment of valence band and impurity levels. We discuss the similarities,\ndiscrepancies, and the limitations of both calculations. We also speculate\npossible reasons for the experimentally observed redshifts of the A and B\nexcitons in the presence of the Sb dopants in MoS$_2$. We hope that these\nresults spark future investigations on other aspects of the problem,\nparticularly those concerning the effects of disorder and electron-hole\ninteraction, and continue to reveal the potential of doped TMDCs for\napplications in optoelectronic devices.\n